The turn of the screw worm

SCREW WORM FLIES are prodigious travellers. And in recent years their movements have taken on a new, and more urgent, significance. Hitching lifts on ships and planes, the American species of the fly has invaded North Africa; its Old World counterpart is just a short hop from Australia. Screw worm flies are not welcome visitors. They are one of the world’s most damaging parasites of livestock: the maggots of the New World screw worm fly ate their way through more than $100 million-worth of American livestock a year in the 1950s and 1960s.

For the past 50 years, researchers have made huge efforts to develop ways to control the flies. Control programmes have worked well in North America, where the fly has now almost disappeared, but other countries have not been so lucky. In 1988, the American species of screw worm fly was accidentally introduced to Libya, where it has done enormous damage. During its first year in Libya, the parasite infested more than 200 people and 2000 domestic animals . Such an accidental introduction of the Old World species to Australia, which is only 110 kilometres from infested New Guinea, could devastate the livestock industry there.

Screw worm flies are one of a group of parasites that evolved to fill a particularly nasty niche in the animal world. When warm blooded vertebrates underwent their explosive diversification some 100 to 200 million years ago, many invertebrates evolved in concert, enabling them to exploit this new and growing supply of food. Some of the true flies (Diptera), the bot flies, warble flies and blow flies, developed maggot larvae that were able …